I have TSA PreCheck but it doesn't always print out on my Southwest boarding pass. This happens almost half the time. Is there anything I can do?
You need to add your TSA Known Traveler Number when checking in, otherwise it's by random choice.
If you have a Rapid Rewards account, add it in the account once, and any flights booked (afterwards) from the account will have it automatically.
To use TSA PreCheck, all these must be in place: a) the airline must have contracted with TSA to allow it, and b) the traveler must have a KTN or be selected randomly and have "TSA PreCheck✓" appear on the boarding pass, and c) the specific airport involved must have PreCheck lanes.
Not all airports used by Southwest have PreCheck lanes. See this Southwest FAQ page, which says:
Will all Southwest airports have a TSA PreCheck lane? No, not all airports have a TSA PreCheck lane, and those that do might not have TSA PreCheck at every checkpoint. Learn more about participating Southwest Airlines airports or for more information on checkpoint locations and hours of operation, visit tsa.gov.
If your TSA Pre Check is through the CLEAR program, you don't have a membership number, or a traveler number. They use biometrics - finger printing to sign into their security line.